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This study provides an algorithm for the rapid on-line transient instability risk assessment associated with existing or forecasted operation conditions. The increasing need to balance the system stability level with its operation costs is the main motivation of the present study. The transient instability risk is defined in terms of the probability of transient instability and its cost. The proposed algorithm is an analytical on-line method making use of three types of information: (i) a transient stability assessment tool, (ii) probabilistic factors of conditional probability theorem and (iii) the cost of transient instability. The risk calculated through the proposed method returns the probabilistic cost of transient instability, and can be used by system operators to improve transient stability with cost-effective changes in the output power of generators. The IEEE New-England 39 bus test system was used to demonstrate the feasibility and applicability of the presented method.